The present invention relates to a system for detecting a pressure difference between the interior of a cabin of an aircraft and the exterior of the aircraft.
In most aircraft, and in particular in passenger aircraft, the interior of the fuselage in general and the cabin in particular is pressurized during flight to maintain the pressure in the cabin equal to a pressure corresponding to a low altitude at which the human body is unlikely to notice a deviation from sea level pressure, even if the aircraft is traveling at a cruise altitude of, e.g., 10,000 meters.
For this purpose, the aircraft is typically provided with a means for continuously pumping air from the exterior of the aircraft into the fuselage and the cabin, and with one or more so-called outflow valves for continuously venting air from the interior of the fuselage and cabin. The ratio of the flow of air into the cabin and the flow of air out of the cabin is adjusted either manually or automatically to thereby obtain and maintain the desired pressure inside the cabin.
In the case of aircraft having gas turbine engines, the means for pumping air into the cabin usually comprises a bleed air supply system which is adapted for taking air from the engines after it has been compressed by the engines but before it is mixed with fuel, and for delivering this air to the cabin. The outflow valves are typically structurally reinforced ports or openings in the aircraft fuselage which can be selectively opened and closed by means of a valve flap.
If an automatic control of the outflow valves is provided, this is effected by means of a cabin pressure control system which operates the one or more outflow valves and usually also controls the inflow of air into the cabin. Even in case of such automated systems, there is usually the possibility of manually controlling the outflow valves in cases of malfunctions of the automatic control.
The desired cabin pressure is set by the cabin pressure control system. Before take-off, the pilot enters the cruise altitude into the cabin pressure control system, which after take-off gradually modifies the cabin pressure in such a manner that abrupt changes are avoided during ascending and that the desired cabin pressure set by the cabin pressure control system is achieved when reaching cruise altitude. For this purpose, the outflow valves, which are fully open on the ground, are gradually closed while the aircraft is climbing.
Prior to the aircraft starting its descent for landing, the altitude of the destination airport is entered into the cabin pressure control system. During the descent, the outflow valves are gradually opened in such a manner that the cabin pressure matches the ground pressure at the destination airport upon touchdown. Thus, if working and operated correctly the cabin pressure control system also serves to equalize the pressure in the cabin with the ambient pressure at the destination airport and prevent the occurrence of a residual cabin pressure after landing at the time the doors or other closing devices for apertures in the fuselage of the aircraft are to be opened. Equalizing the pressure between the cabin and the ambient pressure at the destination airport aids safely opening the doors of the aircraft. An overpressure condition in the cabin is to be avoided because the overpressure could cause the door of the aircraft to swing violently when opened and thereby risking that the person operating the door or closing device or other persons are hurt and that the aircraft or surrounding equipment is damaged.
To prevent such accidents passenger aircrafts are typically provided with means for measuring the cabin pressure and for indicating or displaying the cabin pressure or for warning in case of the occurrence of a potentially dangerous residual cabin pressure, such as, e.g., a pressure difference of greater than 2.5 mbar between the cabin pressure and the ambient atmospheric pressure. Such indicating or warning systems may, in particular, be integrated into the doors or closing devices and may be provided with an autonomous energy device and controller to ensure that they are operable even in case the aircraft has been powered down following reaching the parking position and the on-board power network is therefore switched off partially or completely. The warning may take the form of an acoustic and/or optical alarm provided on or near the door or closing device. If a warning is received or an overpressure condition is indicated, the aircraft crew may simply wait or manually (further) open the outflow valves or a specifically provided valve to allow for rapid pressure equalization.
It is to be noted that residual cabin pressures may occur even if the cabin pressure control system is in principle operating correctly.
As mentioned above, the control system typically relies on the pilot entering the correct altitude of the destination airport, so that the cabin pressure control system is able to appropriately control the outflow valves such that the cabin pressure matches the ambient pressure at the destination airport upon touchdown. Thus, entry of an incorrect altitude may result in a residual cabin pressure.
Further, the cabin pressure control system typically comprises a manual override mechanism. While this mechanism is advantageous for being able to deactivate the automatic operation of an incorrectly operating system or of a system which is not adapted to particular conditions, inappropriate operation can also result in a residual cabin pressure.
Moreover, it is common practice for the crew or ground personnel to manually close the outflow valves while the aircraft is located on the ground, e.g. for preventing entry of animals or dirt through the outflow valves into the fuselage and cabin. In that case the outflow valves no longer function to prevent the buildup of internal pressure on the ground, e.g. due to heating caused by sunlight exposure. To be useful in the latter case, residual cabin pressure indicators or warning devices cannot only rely on the supply of electrical energy from normal energy sources of the aircraft, because these may not be operable at all times while the aircraft is powered down.
Present residual cabin pressure indicators or warning devices include a mechanical pressure sensor, which is operably connected to a tube or pipe system of a different aircraft system, which utilizes the tube or pipe system and is adapted and operable for detecting and determining flight critical data, such as, e.g., altitude and velocity. This tube or pipe system comprises two external pressure ports arranged in the fuselage of the aircraft.
Although—different from the system for detecting and determining flight critical data—residual cabin pressure detection is required when the aircraft is on the ground, very demanding requirements must therefore be met by the pressure sensor, which requirements are determined by the requirements to be met by the flight critical system. This also applies to maintenance of the pressure sensor and the residual cabin pressure indicator or warning device relying thereon.